Page-wide array printers typically use one or more printbars to print a composite image on a print medium. In an ink-jet printer, a printbar may comprise one or more printheads with a plurality of nozzles to eject liquid ink onto a print medium. Ink is ejected from each nozzle using a transducer. In a thermal ink-jet printer, the transducer comprises a nozzle resister; in other ink-jet printers the transducer may comprise a piezoelectric element. Continuous ink-jet printing is also possible, wherein ejected ink is directed onto a print medium using an electro-magnetic field. In most cases, the ejection of ink by a transducer is controlled by a voltage signal, sometimes referred to as a firing signal. The firing signal is generated by a print controller or print engine based on image data associated with a print job, i.e. an image to be printed. Commonly, the one or more printbars are mounted above a media transport that transports a print medium along a path under the one or more printbars. In these cases, ink is ejected across a gap formed below the one or more printbars and is deposited on to the print medium carried by the media transport. The locations where ejected ink drops land on the print medium are sometimes called “ink drop locations,” “ink drop positions,” or “printed pixels”.
The use of one or more printbars provides a number of advantages, such as redundancy in a number of nozzles covering a given area, cancellation of ink-drop weight non-uniformities, and reduced ink-flux per printbar. Nevertheless, printing systems with one or more printbars that print multiple images may in some circumstances present registration problems between the images. For example, to print an image of a print job, a first printbar may print a first image or image component on a print medium and a second printbar may print a second image or image component on the same print medium. As a combination of the first and second image produce a composite image representing the image of the print job, it is important that the first and second images are properly aligned on the print medium. In this context, registration is the process of ensuring that the image components of an image to be printed are located on the print medium so as to generate the correct composite image. For example, in certain cases each printbar may print a separate colour component of the set of cyan, magenta, yellow and black (CMYK). These colour components may be printed on top of each other to provide particular resultant colours; if the colour components are not properly aligned a poor quality final image is produced.
Registration may be based on the readings of encoders attached to a mechanical element of a media transport such as a media roller, a drum, or a belt. These encoders convert a position of the mechanical element into an electronic signal. For example, an electric motor that drives an endless belt system or a printer drum may comprise a rotary encoder that outputs an electronic signal representative of the rotation of the motor. In this case, the output electronic signal is used to infer the position of a print medium carried by the endless belt system or printer drum.
While existing solutions enable image components printed in a printbar system to be registered, they may still produce registration errors. These errors lead to poor image quality. It is also difficult to properly align image components without complex and expensive registration systems. These registration systems may be difficult to scale to larger printing systems.